WO2019238580A1

WO2019238580A1 - Method for connecting components made of sic

Info

Publication number: WO2019238580A1
Application number: PCT/EP2019/065021
Authority: WO
Inventors: Albert Kerber; Jürgen KNORR
Original assignee: Siceram Gmbh
Priority date: 2018-06-15
Filing date: 2019-06-07
Publication date: 2019-12-19
Also published as: DE102018114463A1; EP3684743A1; EP3684743B1

Abstract

The invention relates to a method for connecting components made of sintered and, in particular, hot-pressed silicon carbide, comprising the following steps: introducing a sinterable green body of a thermoplastic SiC compound into a connecting region between opposing connecting surfaces of the components; heating the green body and the connecting region to a sintering temperature under a protective gas atmosphere or vacuum; and pressing the components with the heated green body by applying a sintering pressure.

Description

Process for connecting SiC components

The invention relates to a method for connecting components made of sintered and in particular hot-pressed silicon carbide (SSiC).

The connection of ceramic components becomes necessary if either complicated components cannot be produced monolithically or if the purpose of use requires a joining process such as closing a ceramic container with a ceramic lid after filling the container.

To date, only a few joining methods have been available for the material-to-material connection of the ceramic-ceramic pairing of sintered components. For example, Brevier "Technical Ceramics", Fahner Verlag 2003 discloses soldering and gluing processes using solders and adhesives. In the case of adhesives and metallic solders, it is obvious that the joint seam differs in terms of its composition and properties from the material of the components to be joined. This alien joining is a critical weak point, which can question the usability of the jointed body in general if, for example, the high temperature resistance and / or the corrosion behavior of the seam do not correspond to the parameters of the components and therefore do not meet the conditions of use. A laser joining method is also known from US Pat. No. 7,462,25 B2. Although the cohesive connection of components made of sintered silicon carbide (SSiC) is achieved with this modern joining process, the use of solders with meltable parts (glass solder) leads to the formation of silicon carbide in the seam, but it is a liquid-phase silicon carbide ( LPSiC), i.e. with regard to the components around an alien silicon carbide. The LPSiC has significantly poorer properties with regard to high temperature and corrosion resistance than the components to be connected made of pressure-free sintered silicon carbide (SSiC).

At present, no technology is known which enables a specific connection of components made of sintered or hot-pressed silicon carbide in an effective short-term process and at the same time maintains the excellent properties of the components for the joined body as a whole.

It is therefore an object of the present invention to provide a method which ensures a type-specific material connection of components made of sintered or hot-pressed silicon carbide, which is also safe and durable and which only requires a short energy input, largely limited to the joining zone, to other adjacent ones Materials, such as the contents in a container, to be thermally stressed as little as possible.

Against this background, the invention relates to a method for connecting components made of sintered and in particular hot-pressed silicon carbide, with the following steps: introducing a sinterable green body made of a thermoplastic SiC compound into a connection area between opposite connection surfaces of the components; Heating the green body and the connection area to sintering temperature under a protective gas atmosphere or vacuum; and pressing the components with the heated green body by applying a sintering pressure. After the sintering process is complete, cooling can take place.

The task is therefore solved by hot pressing in combination with the use of a green body that is specially matched to the components and the joining process. A type-specific connection of the components is created. A sinterable green body made of thermoplastic SiC material is placed between the SSiC parts to be connected and heated to sintering temperature under protective gas, for example argon, or vacuum, in any case with exclusion of oxygen. By applying an axial force perpendicular to the joint, one-dimensional shrinkage is forced. A uniaxial compression pressure prevents a three-dimensional shrinkage that often occurs during unpressurized sintering, which can cause cracks in the joint seam.

For short-term sintering, inductive hot pressing (IHR) and Spark-Piasma-Sintering SPS or Pulsed Electric Current Sintering (PECS) are known from the prior art. The characteristic of these two sintering processes is the combined action of high temperatures and pressures on the sintered material. So far, however, both processes have only been used to produce sintered components from powdered starting materials. According to the invention, aspects of these methods can be used in the connection of parts that have already been sintered.

The green body introduced into the connection area has a shape which is matched to the joining surface geometry and surface quality of the joining surfaces of the components. The green body is preferably in the form of a thin layer in the form of a seam foil. The thickness of the seam foil can be, for example, 1 mm or less. In one embodiment, the green compact is partially debindered before it is introduced between the components. The binder content should be such that flexibility and thus manageability and adaptation to the connection surfaces of the components are retained.

In one embodiment, green compacts with a green density of between 1.5 and 2 g / cm ³ , preferably between 1.6 and 1.9 g / cm ³ are used.

In one embodiment it is provided that the sintering temperature is 1800-1950 ° C. and / or that the sintering pressure is 20-40 MPa and / or that the sintering pressure and sintering temperature are maintained for 5-15 minutes. Pressurization preferably begins before the sintering temperature is reached, but after the first phase. The connection between the components is formed during the sintering phase under sintering conditions (pressure and temperature).

In one embodiment it is provided that the heating of the green body comprises at least two phases, the green body being heated in a first phase to a degassing temperature below the sintering temperature and only in a second phase to the sintering temperature, preferably providing that the connection area is evacuated during the first phase of heating and protective gas is applied during the second phase. The two phases can follow one another in the course of a constant rise in temperature, but it can also be provided that the temperature is not constantly increased but is kept at a degassing temperature below the sintering temperature for a certain period of time. It can be provided that the joining zone is initially heated with partial load in order to support the outgassing of the green body and the connection areas in a vacuum. Suitable residual pressures for the vacuum include 0.5-2 mbar. The degassing temperature can be, for example, 300-800 ° C. The evacuation is then ended and the reaction space is filled with protective gas. suitable Shielding gas pressures are, for example, between 1-2.5 mbar. Further heating is now carried out at full load.

In one embodiment it is provided that the green body is fixed between the connection surfaces before heating. The process is based on the insertion of a sandwich arrangement consisting of the first component, the green body and the second component in a press and a gentle pressure relief to fix the geometry.

In one embodiment it is provided that the green body is heated by means of induction. Inductive heating enables the concentration of thermal energy to be concentrated on the seam and its immediate surroundings. The duration of the exposure can be kept short.

Alternatively or additionally, the heating can also be carried out by means of resistance heating.

In one embodiment it is provided that the green body is heated using a susceptor. A susceptor is a component made of a material that is located in the immediate vicinity of the green body and can absorb electromagnetic energy and convert it into heat. A suitable material includes graphite, but alternatively, for example, stainless steel, molybdenum, aluminum or SiC are also suitable. The susceptor can be used, for example, in the form of a ring or a disk. If the method is used to close a container or to connect pipe flanges, it is conceivable that a susceptor is arranged on the side of the components facing away from the heat source or coil.

In one embodiment it is provided that during the pressing a preferably pulsed DC voltage across connecting surfaces of the components and the green compact is laid. Applying a pulsed DC voltage across the joining zone can increase the reactivity at the connection surfaces and thus accelerate the sintering process and increase the strength of the connection.

The invention further relates to the use of a method according to the invention for closing a container with a lid, wherein at least the contact surfaces of the container and lid consist of sintered or hot-pressed silicon carbide (SiC). The container and lid preferably consist entirely of sintered or hot-pressed silicon carbide (SiC). Use on containers and lids comprising or consisting of sintered SiC (SSiC) can be particularly preferred. If the container is filled with a material, a thermal shield can be placed between the filling material and the joining area in order to protect the filling material from thermal stress.

In one embodiment it is provided that the container contains radioactive or toxic material. The method can be used in particular for the final storage of radioactive waste in containers made of sintered or hot-pressed silicon carbide (SiC). The containers can be filled with the radioactive material and closed with a lid, the container and lid being connected using a method according to the invention. The sealed containers can then be stored in a final deposit.

Further suitable possible uses for the method according to the invention include connecting pipes with pipe flanges, two or more plates, a wheel and a shaft or the like, at least the contact surfaces being made of sintered or hot-pressed silicon carbide (SiC).

Further details and advantages of the invention result from the exemplary embodiments illustrated below with reference to the figures. The figures show: Figure 1: the structure of an exemplary system for performing a method according to the invention;

Figure 2: the construction of an alternative system for performing a method according to the invention;

Figure 3: the structure of an exemplary system for closing a container using a method according to the invention;

Figure 4: the construction of an alternative system for closing a container using a method according to the invention;

Figure 5 is a schematic representation of the process control of the method according to the invention in a system according to Figure 2;

FIG. 6: Examples of different component geometries that can be connected using the method according to the invention; and

FIG. 7: microscopic images of a joint seam produced as part of a method according to the invention.

Figure 1 shows the structure of an exemplary system for performing a method according to the invention.

The seam film N is placed between the components A and B to be joined and the joining geometry is fixed by a slight pressure between the upper punch 2 and the lower punch 3. The direction of the pressing force is illustrated by the arrows 4. The joining zone is heated directly via the induction coil 1. Since the sintering process requires an inert gas atmosphere at low gas pressure, the inert gas chamber 5 encloses the entire joining zone and the system components mentioned. Shielding gas can be introduced through shielding gas connection 5a. Another connection 5b is used for evacuation and ventilation. Both connections 5a and 5b are provided with valves.

Figure 2 shows the structure of an alternative system for performing a method according to the invention. Corresponding parts are provided with identical reference symbols.

In this variant, the joining zone is indirectly heated by heat conduction from suitably arranged susceptors 6, which in turn are heated, for example, directly inductively using induction coil 1 or by current flow. A pulsed DC voltage 7 can be applied across the joining zone to accelerate the initiation of the sintering process.

Figure 3 shows the basic structure of an exemplary system for closing a sintered container A with the sintered lid B using the inventive method. Corresponding parts are again provided with identical reference symbols.

The container comprises filling material 9. The shape and thickness of the seam foil N is adapted to the joint surface geometry and the surface properties of the joint surfaces (roughness depth, parallelism). In the simplest case, the joining zone is again heated directly inductively.

Special applications can make an expansion of the structure according to FIG. 4 advantageous. Corresponding parts are again provided with identical reference symbols.

In order to keep the sintering time short, the joining zone is heated both directly via the induction coil 1 and indirectly via heat conduction from the susceptors. ren 6, which in turn are heated by the induction coil 1. A pulsed DC voltage 7 applied across the joining zone can be helpful for accelerated initiation of the sintering process, combined with an additional energy input for heating. The thermal shield 8 serves to protect the filling material 9 against impermissible thermal loads.

FIG. 5 schematically shows the process control of the method according to the invention in a system according to FIG. 2.

The initial situation is characterized by an open protective gas chamber 5, which is filled with air from the ambient pressure pU. The press rams are open, the press pressure P is zero. The inductive heating is switched off. The components A and B to be joined have the ambient temperature TU. The joining surfaces of A and B were hard machined and have a given roughness depth and plane parallelism to each other. The roughness depth influences the degassing process of the seam foil, the surface activation and the electrical contact resistance at the interfaces between the seam foil and the components A and B. Roughness depth and plane parallelism determine the thickness d of the seam foil to be selected, which is for example 1 mm or less.

The process begins with the insertion of the sandwich arrangement consisting of component A, the seam film N and component B between the lower temple 3 and upper punch 2, the approach of the upper punch and a gentle pressure load PG to fix the geometry.

The protective gas chamber is then closed and evacuated. The inductive heating of the joining zone begins with partial load to support the outgassing of the seam foil. At pressure pE, the evacuation is ended and the protective gas chamber is filled with protective gas, for example argon, up to pressure pS. Further heating is now carried out at full load. Even before reaching the isothermal Sintering temperature TS, the uniaxial pressure loading takes place with the sintering pressure PS.

To initiate the sintering process, especially at the interfaces (docking phase), a pulsed DC voltage DC is optionally switched on. The heating of the joining zone is regulated in such a way that isothermal sintering is guaranteed at temperature TS. The actual sintering phase tS is relatively short, for example 10 minutes. It is characterized by the establishment of the connection between components A and B and the one-dimensional sintering shrinkage of the seam foil N. After the sintering phase has ended, the heating is switched off. The cooling phase ends when the ambient temperature TU is reached with the pressure relief of the component (P = 0) and ventilation of the protective gas chamber (p = pU). The total cycle time tZ is, for example, one hour or less.

The method according to the invention can be used to connect different component geometries. Figure 6 shows some examples. Figure 6a shows a sandwich arrangement with a seam film N, which is enclosed between the components A and B to be connected. Susceptors 6 are arranged on the outer sides of components A and B. Figure 6b shows a container (component A) with a lid (component B) in side view and sectional view. Figure 6c shows a pipe with flange in side view and sectional view, wherein the flange of a tube can represent component B and the tube component A. Figure 6d shows a stacked arrangement of several plates, corresponding to components A, B, C, D in side view and sectional view. Such structures can be used, for example, as heat exchangers or in fuel cells. Figure 6e shows a shaft (component A) with wheel (component B) in side view and sectional view. The connection produced using a method according to the invention can withstand high torques.

Embodiment: The exemplary embodiment 1 relates to a component geometry according to FIG. 6 a in a system according to FIG. 1 with additional susceptors 6.

Components A and B are octagons made of sintered silicon carbide (SSiC) with an outer circle diameter of 37 mm and a height of 3.5 mm. The joining surfaces were not machined but remained as if sintered. The seam film consists of a thermoplastic SiC compound with a uniformly high green density and is partially debonded to the extent that flexibility and thus manageability and adaptation to the joining surfaces are retained. A sandwich arrangement was stacked from a graphite susceptor 6, component A, seam foil N, component B and a further graphite susceptor 6 and clamped between lower punch 3 and upper punch 2. In the green state, the seam film N had a thickness of 0.5 mm and a green density of 1.701 g / cm ³ .

The joining process then started as shown in FIG. 5. The protective gas chamber was evacuated to a residual pressure of pF = 1 mbar to ensure that the seam film was largely degassed. The chamber is then filled with argon up to a pressure of pS = 2.0 mbar. The isothermal sintering took place at 1850 ° C. over a period of 10 minutes. The uniaxial pressure P was 30 MPa. Due to the one-dimensional shrinkage of the seam foil, the thickness of the sintered joint seam was approx. 0.27 mm. The seam was free of pores and cracks with good adhesion to the joining surfaces of the components, as can be seen from the microscopic images shown in FIG. 7.

The use of a pulsed DC voltage was not necessary in the example described, but could be viewed as an option reserve for a further shortening of the isothermal sintering phase.

Claims

Expectations

1. Method for connecting components made of sintered and in particular hot-pressed silicon carbide, with the following steps:

Introducing a sinterable green body made of a thermoplastic SiC compound into a connection area between opposite connection surfaces of the components;

Heating the green body and the connection area to sintering temperature under a protective gas atmosphere or vacuum; and

Pressing the components with the heated green body by applying a sinter pressure.

2. The method according to claim 1, characterized in that the green body introduced into the connection area has the shape of a thin layer, preferably with a thickness of 1 mm or less.

3. The method according to any one of the preceding claims, characterized in that the sintering temperature is 1800-1950 ° C and / or that the sintering pressure is 20-40 MPa and / or the sintering pressure and sintering temperature are maintained for 5-15 minutes.

4. The method according to any one of the preceding claims, characterized in that the heating of the green body comprises at least two phases, the green body being heated in a first phase to a degassing temperature below the sintering temperature and only in a second phase to the sintering temperature, preferably provided is that the connection area is evacuated during the first phase of heating and protective gas is applied during the second phase.

5. The method according to any one of the preceding claims, characterized in that the green body is fixed between the connecting surfaces before heating.

6. The method according to any one of the preceding claims, characterized in that the green body is heated by resistance heating.

7. The method according to any one of the preceding claims, characterized in that the green body is heated by means of induction.

8. The method according to claim 7, characterized in that the green body is heated using a susceptor.

9. The method according to any one of the preceding claims, characterized in that a preferably pulsed DC voltage is placed over connecting surfaces of the components and the green body during pressing.

10. Use of a method according to one of the preceding claims for closing a container with a lid, wherein at least the contact surfaces of the container and lid consist of sintered or hot-pressed silicon carbide (SiC).

11. Use according to claim 10, characterized in that the container contains radioactive or toxic material.